Accessing a render in a remote GUI instance
After your single image render has completed, there should be an EXR file in your System Temp folder, even if Blender has not formally outputted a frame image. However this is only the case if you have enabled Save Buffers in the Render Performance panel beforehand.

Check your Blender preferences for where that folder is on your Linux system (default path is /tmp for linux). The folder may be hidden, so you may have to unhide it to see it, but there should be an EXR file there until a system reboot or the folder is cleared for some other reason.

Alternatively, if your Blender app is running in GUI mode, you could possibly Remote Desktop into your Linux machine, and just save the image out of Blender.

Failing those 2 solutions, you may be out of luck, unfortunately.